Alkaline bath removal of scale from titanium workpieces

ABSTRACT

TITANIUM SCALE, GENERALLY COMPRISING OXIDES AND NITRIDES OF TITANIUM, MAY BE REMOVED FROM TITANIUM WORKPIECES BY IMMERSING THE WORKPIECE IN AN ELEVATED TEMPERATURE AQUEOUS BATH BETWEEN ABOUT 180*F. AND 200* F. CONTAINING PER GALLON FROM 3.1 TO 5 POUNDS OF THE MIXTURE CONSISTING ESSENTIALLY OF SODIUM GLUCONATE, 8-30 WEIGHT PERCENT, TRIETHANOLAMINE, 3-24 WEIGHT PERCENT AND SODIUM HYDROXIDE, BALANCE TO 100 WEIGHT PERCENT, AND REMOVING THE WORKPIECE FROM THE BATH FOLLOWING REMOVAL OF SCALE FROM THE WORKPIECE.

United States Patent 3,553,015 ALKALINE BATH REMOVAL OF SCALE FROMTITANIUM WORKPIECES Charles L. Dohogne, San Pedro, Califi, assignor toPurex Corporation, Ltd., Lakewood, Califl, a corporation of CaliforniaNo Drawing. Filed June 30, 1969, Ser. No. 837,950 Int. Cl. C23g 1/22 US.Cl. 134-2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION(1) Field of the invention This invention has to do with the removal ofscale, such as oxides, nitrides, sulfides and the like from the surfacesof titanium workpieces. Titanium scale and particularly oxides oftitanium constitutes virtually the most refractory of all knownmaterials. Titanium oxides have very high melting points and areresistant to almost all types of chemical attack. Many uses of titaniumrequire the removal of the natural oxide coating or an oxide coatinginduced by metal treating processes such as heat treating, anodizing oroxides produced in the use of a part.

(2) Prior art The heretofore accepted method for descaling titanium andremoving the oxides from workpiece surfaces has included an alkalineprecleaning step, sometimes followed by an alkaline conditioning stepand a post treatment in a bath of nitric-hydrofluoric acid. Titaniumoxide has been thought to be impervious to alkaline solutions andaccordingly, efforts at improving the cleaning of titanium workpieceshave been directed primarily at the development of more vigorous andmore effective acids.

SUMMARY OF THE INVENTION It has now been discovered that a highlyalkaline solution such as an aqueous solution of sodium hydroxide iseffective to remove scale from titanium workpieces where the scaleremoving bath contains certain amounts of both triethanolamine andsodium gluconate. The sodium gluconate may be formed in situ by thereaction of gluconic acid and sodium hydroxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the presentinvention, a treating bath is prepared by dissolving in water from 3.1to 5 pounds and preferably from 4 to 5 pounds of a mixture of sodiumgluconate, triethanolamine and sodium hydroxide. The bath is heated toelevated temperature, generally about 175 F. up to just below theboiling point of the aqueous bath, and preferably between about ISO-200F.

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The mixture dissolved in the aqueous bath consists essentially of sodiumgluconate at 8 to 30 weight percent and preferably 13 to 23 weightpercent, triethanolamine at 3 to 24 weight percent and preferably at 8to 18 weight percent and the balance to percent of sodium hydroxide.

The workpiece which may be degreased by conventional procedures inadvance of scale removal, is immersed in the aqueous bath having thecomposition indicated above. Scale removal is initiated almostimmediately with hydrogen evolvement occurring visibly after a period ofabout one minute. Treatment in the bath is continued for a sufficientperiod to remove the scale from the workpiece surface. This may rangefrom one minute to five minutes or more, up to about thirty minutes.Multi le immersions in successive or the same aqueous bath can be usedwhere desired.

The titanium workpiece removed from the bath is cleaned of scale and issuitable for use subsequently in welding, bonding, plating or painting.Thus, the aqueous bath herein disclosed serves as a cleaner and a picklefor titanium.

Titanium alloys can be similarly treated as titanium workpieces. Alloyswith such materials as columbium, hafnium, zirconium, and tantalum maybe cleaned by the indicated procedures.

EXAMPLE 1 A titanium scale removal bath was prepared as follows: Amixture of 18.5 percent sodium gluconate, 13.5 percent triethanolamine,and correspondingly 68 percent sodium hydroxide was mixed dry. Anexothermic reaction took place with production of water. The mixture wasdissolved at the rate of 3.1 to 5 pounds per gallon of bath in water.The water' solution was heated to 200 F. Titanium workpieces havingoxides and possibly nitride scale on their surfaces resulting from heattreating were immersed in the bath for periods ranging from ten minutesto thirty minutes. The workpieces were removed from the bath, when thescale thereon had been removed, rinsed and dried. The workpieces werenoted to be clean enough without further processing for welding,bonding, plating or painting operations.

Control A Example 1 was duplicated but omitting the sodium gluconate.There was no appreciable removal of scale in an hour and a half.

Control B Example 1 was duplicated employing a mixture of sodiumhydroxide and sodium gluconate. The resulting bath attacked the titaniumoxide, but at a slow rate, requiring ninety minutes for removal. Inaddition, smut was produced on the titanium surface whichwas removableonly with a dip in acid.

Control C Example 1 was duplicated but omitting the sodium hydroxide.The resulting bath was inelfective for removing titanium scale fromtitanium workpieces.

Accordingly, combination of any two of the cleaning bath components isnot effective as a cleaner due to slowness of attack on the substrateand oxide and uneconomically long processing times.

In the typical practice of the present invention, immersion times willrange between one minute and ten to thirty minutes with longer orshorter periods, depending on scale conditions, temperature of the bath,and concentration of the bath being useful. In this connection, arelatively more concentrated bath e.g. containing from 4 to 5 pounds ofthe above described mixture may be employed where particularly obduratescale is to be removed.

I claim:

1. A method of descaling titanium comprising immersing a scaled titaniumworkpiece in a bath heated above 175 F., said bath being a solution offrom 3.1 to 5 pounds per gallon of a mixture consisting essentially ofWeight percent Sodium gluconate 8-30 Triethanolamine 3-24 Sodiumhydroxide Balance to 100 4 4. Method according to claim 1 in which saidmixture contains Weight percent Sodium gluconate 13-23 Triethanolamine8-18 Sodium hydroxide Balance to 100 5. Method according to claim 4 inwhich the workpiece is immersed in the bath for a period of 10 to 30minutes.

References Cited UNITED STATES PATENTS 4/1963 Netzler l34-27 12/1969Moll l5620X US. Cl. X.R.

